Cost Comparison of Conventional Gray Combined Sewer Overflow Control Infrastructure versus a Green/Gray Combination

Abstract

This paper outlines a life-cycle cost analysis comparing a green (rain gardens) and gray (tunnels) infrastructure combination to a gray-only option to control combined sewer overflow in the Turkey Creek Combined Sewer Overflow Basin in Kansas City, MO. The area of this basin is 2,248 ha (5,554 acres). A set of spreadsheets was developed upon which this life-cycle cost analysis is based. The spreadsheets allow the user to vary the length and/or the diameter of the tunnels. The spreadsheets calculate the additional number of rain gardens needed to handle a given amount of stormwater when the tunnel sizes are reduced. The difference in life-cycle costs between the green/gray and gray-only alternatives is calculated. For a range of options for the number of rain gardens, the green/gray combined alternative is more cost-effective than the gray-only option. The potential present worth of the cost savings for the 50-year expected lifespan of both the gray and green infrastructure could reach as much as $35 million for the Turkey Creek Basin.